They can only choose to do that if they can run that software. We are pretty close to the situation already where users no longer have choice over what software they can run.

The iPhone went there years ago. The situation on Android is tenuous. OSX and Windows both now both start from a secure boot mechanism. At this point it is still possible for a user to work around these things but this is now "at the pleasure" of the OS maker and not something the user actually controls. Removal of a couple of checkboxes is how far we are from losing user-controlled right to install software on any mainstream consumer supported OS.

So I am arguing that now that operating system manufacturers have forfeited their ability to say they cannot prevent the user from installing software, it is only a matter of time before it becomes legally mandated that they close off this capability, and deliver only software that is delivered from a verified (probably government licensed) channel.